Lightwave communications systems applied in the field of telecommunications can be broadly classified into two categories. These two categories are referred to as long-haul and short-haul systems, depending on whether the optical signal is transmitted over relatively long or short distances compared with typical intercity distances (approximately 50 to 100 kilometers). Long-haul communications systems require high-capacity trunk lines and can transmit information over several thousands of kilometers using optical amplifiers.
Long-haul communications systems are used to carry international communications traffic from one continent to another. Since this often requires the laying of fiber trunk lines underwater, these systems are often referred to as submarine systems.
In submarine systems, as well as terrestrial systems, it becomes necessary to direct certain wavelengths of wavelength-multiplexed optical signals carried on these high-capacity fiber trunks. This typically occurs to conform to desired traffic routing parameters.
The optical component used to redirect these signals is referred to as an optical add-drop multiplexer (ADM). An ADM is known as a key device for use in splitting and inserting wavelength-division multiplexed optical signals.
Undersea optical communication systems include transmitter and receiver terminals connected by a fiber transmission medium and repeaters containing optical amplifiers that compensate for attenuation in the fiber. To provide increased flexibility in undersea network architecture beyond simple point to-point interconnects, a branching unit is provided, which allows traffic to be split or switched to multiple landing points, which are referred to as branch terminals. The branching unit contains the ADM that redirects the optical signals from the trunk connecting the transmitting and receiving terminals to the branch terminal. In addition to serving as optical interconnects, branching units also provide and manage electrical power to the repeaters. In wavelength division multiplexed communication systems, the branching unit drops selected wavelengths or channels to the branch terminals while transmitting the remaining wavelengths that compose the WDM signal.
U.S. Appl. Ser. No. 08/728,591 discloses a branching unit which drops optical information signals of selected wavelengths received from a transmitting trunk terminal to a branch terminal. This known branching unit transmits all the wavelengths of a WDM signal except for those information signals carried at the selected wavelengths that are to be added or dropped to the branch terminal. The branch terminal replaces the received optical information signal at each selected wavelength with another optical information signal. The new optical information signal, carried at the selected wavelength, is transmitted by the branch terminal to the branching unit, which in turn multiplexes the selected wavelength onto the WDM signal. The end result is that the branching unit drops certain information signals while receiving additional optical information signals to replace the dropped signals. The additional information signal is then carried along with the other WDM signals.
However, one limitation of this branching unit is that the selected wavelength to be added/dropped cannot be changed for a given branching unit. Moreover, the same branching unit cannot process different combinations of two of more selected wavelengths in an add/drop mode.
It therefore would be desirable to provide a single branching unit in which any combination of selected wavelengths to be added/dropped can be arranged.